Need a long Allen/hex to remove/install brake levers but the short end of the key then doesn't give you enough leverage to actually loosen/tighten the bolt? Just put a socket (the deeper the better) on the hex and attach whatever extension you want. Voilą, there's your leverage.
(Note: If you don't have a socket of the size you need, just use the next size up. A bit of play is not a problem.)
Need a cheap 17-mm cone wrench/spanner? Go into any hardware store and ask for a replacement wrench/spanner for holding the spindle on an angle grinder (NOT the pin spanner for the locknut!). All I've seen so far were 17s and about 2 mm thick.
Want to tighten BB cups but your torque wrench only works in one direction? Put an extension on it and push it through the BB, attaching it to the BB cup tool from the inside. Your torque wrench's right is now the cup's left. This will obviously only work on BBs for 2-piece cranks (i.e. with the spindle on the crankset itself), though.
Your chain whip slips on the cassette, or you don't have a whip to begin with? Get an impact wrench/driver on the remover instead of a breaker bar/ratchet, set it to reverse, hold the cassette with a thick glove or a rag, and press the trigger. The impact action will remove the locknut very quickly with very little torque on the cassette and no risk of cutting yourself on the teeth. You can also press down on the gun to keep the remover seated in the splines.
The method mentioned above can, of course, also be used on freewheels. The only difference is that you need to hold the wheel instead of the cassette.
You can use a vacuum pump (sold as an automotive brake bleeding tool,
here's one example) to bleed brakes without making a mess (I've only tried my Shimano M615s, but it should work on other models, too). Attach a funnel with brake fluid/oil to the lever and pump the lever a few times to get all the air from the lever out, then connect the vacuum pump to the bleeder valve on the caliper with a piece of clear tubing, pull a vacuum and open the bleeder about half a turn. Brake fluid/oil will be drawn from the funnel on the lever into the caliper, taking any air bubbles with it. There are two big advantages to this method: The first is that there's no positive pressure to push the hose off the bleeder nipple. And if it does come off, any fluid in it will be sucked into the reservoir attached to the vacuum pump and the rest will stay in the caliper instead of spraying everywhere. The second is that once you have the vacuum and the bleeder open you can sit the pump on something and watch the brake bleed itself. All you need to do after that is to watch the level of fluid in the funnel and close the bleeder when there's still a bit left so that you don't draw air into the lever (but even if you do, you'll only have to "burp" the lever, not bleed the whole thing).
And the last tip for today is related to the above: Instead of using the funnel from Shimano, you can use a regular syringe with the plunger removed. The end (where the needle goes) is just the right size so that you can screw it into the lever and it'll stay in there perfectly and not even leak (despite not having the O-ring that the Shimano funnel has). Not only is it cheaper, it's also transparent so you can see how much fluid/oil is in it from any angle. Just keep a rag on it when you're removing it to catch the oil that's still in it (which won't be more than about 2 ml if you use the method described above).